1. Field of the Invention
The present invention relates to highly soluble aromatic polyimides wherein the dianhydride used to form the polyimide is at least partly 6FDA. The remaining portion of the dianhydride-derived polyimide is derived from several of the more common aromatic dianhydrides, as defined below, used to form polyimides. The diamine portion of the polyimide is derived essentially from amine terminated siloxane units.
2. Prior Art
As taught in the prior art, siloxane-containing polyimides can be prepared by, for example, reacting a siloxane-containing diamine with a dianhydride. The initial product of such reactions, a polyamide acid, has been found to be soluble in highly polar solvents, such as N-methyl pyrrolidone. Solutions of such polyamide acids have been typically used to coat substrates. Such coatings have been converted to the siloxane-containing polyimide by heating, usually between 150.degree. C. and 400.degree. C., to remove the solvent and to effect cyclization of the polyamide acid. These processes are complicated by further problems, such as void formation caused by the evolution of the by-product water during the cure step and the like.
These polyimides, while useful as protective coatings for semi-conductors and other electronic devices, suffer from the shortcoming of being insoluble in most low boiling organic solvents. They suffer from the shortcoming that many semiconductor devices cannot be heated to the extreme temperatures required to effect the cyclization of the precursor polyamide acid (150.degree. C.-400.degree. C.) as discussed above. Further, it has been taught in the prior art that such polyamide acids are unstable to hydrolysis. Such hydrolysis would tend to degrade the utility of the final product. Because of these and other shortcomings, it would be highly desirable to have siloxane-containing polyamide materials which are soluble in low-boiling solvents.
These shortcomings have been partially overcome in the prior art. Berger, in U.S. Pat. No. 4,395,527, discloses that polyimides incorporating a siloxane unit of formula ##STR1## where Q is a substituted or unsubstituted aromatic group; Z is ##STR2## D is an unsubstituted or substituted hydrocarbylene; R.sup.1 R.sup.2, R.sup.3, R.sup.4, R.sup.5, and R.sup.6 are each independently unsubstituted or substituted hydrocarbyl; X, Y and Z each independently has a value from 0 to 100; have improved solubility parameters. For example, these polyimides are taught to be soluble in chlorinated hydrocarbon solvents such as dichlorobenzene and trichlorobenzene and in good polar solvents such as N, N-dimethyl acetamide, N-methyl caprolactam, dimethylsulfoxide, N-methyl-2-pyrrolidone, tetramethyl urea, pyridine, dimethylsulfone, hexamethyl phosphoramide, tetramethylene sulfone, formamide, N-methylformamide, butyrolactone and N-acetyl-2-pyrrolidone (U.S. Pat. No. 4,395,527, column 28, line 65). However, one shortcoming found for these materials is their lack of solubility in very weak solvents, such as toluene. Further, as one knowledgeable in the art would appreciate, this reference requires the use of unusual monomers which are not readily available.
Lee, in U.S. Pat. No. 4,558,110, discloses crystalline polyimides which incorporate polydiorganosiloxane units terminated with amine functions. These materials are found to be soluble in ortho-dichlorobenzene but not soluble in good aprotic solvents, such as N-methyl pyrrolidone.
Incorporation of bis(aminoalkyl)siloxane components into polyimides has not been shown to be a sufficient criteria for polyimide solubility. The prior art teaches that such materials suffer from the shortcoming of being insoluble in most low boiling organic solvents (see, for example, U.S. Pat. Nos. 4,395,527, 4,480,009, 4,449,149, 4,586,998, 4,609,569, and U.S. Pat. No. 4,652,598). Lee, in U.S. Pat. No. 4,558,110 discloses crystalline polyimides which contain bis(amino alkyl)-terminated siloxanes. These materials were found to be soluble in halogen-containing solvents, such as ortho-dichlorobenzene. However, these materials were not found to be soluble in even very good aprotic solvents, such as N-methyl pyrrolidone. No solubility of these materials in very weak aprotic solvents, such as toluene, was reported.